If my understanding is correct, a GPS satellite produces a pretty simple signal which is basically composed of its location and its time. Given 4 of these signals, then one can uniquely solve for the X,Y,Z position (and as a by product, time) of the hand-held GPS unit.
Why don't we use use cell towers like we use GPS satellites?
It's much more easy to tell the position of a cell tower than a satellite (they don't move). And they can be given atomic clocks just like GPS satellites are given. Then we would have more redundancy, more availability, and more accuracy in determining location.
Note: I know that the E911 service uses cell tower to triangulate the position of cell phones, but this technology is based upon measuring the signal strength of the tower and is therefore nowhere nearly as accurate as GPS.